BACKGROUND AND AIMS: To form nitrogen-fixing nodules on pea roots, Rhizobium leguminosarum biovar viciae must be competitive in the rhizosphere. Our aim was to identify genes important for rhizosphere fitness. METHODS: Signature-tagged mutants were screened using microarrays to identify mutants reduced for growth in pea rhizospheres. Candidate mutants were assessed relative to controls for growth in minimal medium, growth in pea rhizospheres and for infection of peas in mixed inoculants. Mutated genes were identified by DNA sequencing and confirmed by transduction. RESULTS: Of 5508 signature-tagged mutants, microarrays implicated 50 as having decreased rhizosphere fitness. Growth tests identified six mutants with rhizosphere-specific phenotypes. The mutation in one of the genes (araE) was in an arabinose catabolism operon and blocked growth on arabinose. The mutation in another gene (pcaM), encoding a predicted solute binding protein for protocatechuate and hydroxybenzoate uptake, decreased growth on protocatechuate. Both mutants were decreased for nodule infection competitiveness with mixed inoculants, but nodulated peas normally when inoculated alone. Other mutants with similar phenotypes had mutations predicted to affect secondary metabolism. CONCLUSIONS: Catabolism of arabinose and protocatechuate in the pea rhizosphere is important for competitiveness of R.l. viciae. Other genes predicted to be involved in secondary metabolism are also important.

Department of Molecular Microbiology John Innes Centre Norwich Research Park Norwich NR4 7UH UK

Department of Molecular Microbiology John Innes Centre Norwich Research Park Norwich NR4 7UH UK ; Department of Plant Sciences University of Oxford South Parks Road Oxford OX1 3RB UK

Department of Molecular Microbiology John Innes Centre Norwich Research Park Norwich NR4 7UH UK ; Institute of Microbiology Academy of Sciences of Czech Republic Videnska 1083 142 20 Prague Czech Republic

School of Biological Sciences University of Reading Reading RG6 6AJ UK

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